/**
 * #      671
 * 名称   二叉树中第二小的节点
 * 链接   https://leetcode-cn.com/problems/second-minimum-node-in-a-binary-tree
 * 题解   [318](https://leetcode-cn.com/problems/second-minimum-node-in-a-binary-tree/solution)
 * 通过率 46.2%
 * 难道   简单
 */

